Ink jet printing has become increasingly common as a method for rapid printing of high-definition characters and symbols. In one application, an ink jet printer is used to print optically-scanable bar codes. See, for example, Rogers U.S. Pat. No. 4,714,934, issued Dec. 22, 1987 and Kniepkamp U.S. Pat. No. 4,819,009, issued Apr. 4, 1989. Such bar codes must be printed with both high contrast and high clarity to ensure that scans succeed 99.9% of the time. This is relatively easy on ordinary paper, but becomes more difficult on rough surfaces such as paperboard due to surface irregularities and wicking of the ink. Thus, the printing of bar codes directly on the sides of paperboard cartons requires an ink which resists wicking and which provides a clear, precise image.
Oleate bases are widely used in the ink industry as a means of solvating dyes and producing liquid colorants that can be augmented and used in a variety of printing applications, including writing and printer ribbon inks. Unfortunately, oleic-acid based inks generally give poor print quality due to severe bleed and ink spread on porous papers and substrates. Spread of the imprint also causes the print to lose intensity and look more grey. Bar coding applications require a predetermined minimum intensity of the imprint for scanner recognition.
A variety of specific ink compositions containing oleic acid and other fatty acids and fatty acid esters are known. Ink jet inks based on oleic acid and other fatty acids, generally in combination with an alcohol or glycol, are described in Lin U.S. Pat. No. 4,531,976 issued Jul. 30, 1985, Cooke et al. U.S. Pat. No. 4,361,843, issued Nov. 30, 1982, and Merritt et al. U.S. Pat. No. 4,484,948, issued Nov. 27, 1984. Nigrosine dyes have been used in connection with ink jet ink formulations containing oleic acid; see Lin et al. U.S. Pat. No. 4,822,418, issued Apr. 18, 1989.
Lin U.S. Pat. No. 4,386,961, issued Jun. 7, 1983, describes an ink containing 1-10 wt. % oleic acid, 30-45 wt. % diethylene glycol or a (poly).sub.n ethylene glycol, wherein n is from 1 to 5, and 37-45 wt. % of a methoxy triglycol as a common solvent. However, the dyes disclosed are the type which dissolve in the solvent and do not react with the oleic acid to form an ester.
The use of antioxidants in oleic acid-based inks is also known. Antioxidants have been used in certain types of fatty acid-based ink jet inks; see Lin et al. U.S. Pat. No. 4,793,264, issued Dec. 27, 1988. Such antioxidants have been used to prevent corrosion of the ink jet nozzles.
Despite the variety of fatty acid-based ink formulations available, a need remains for ink jet ink compositions having superior printing characteristics, particularly for use in printing bar codes on rough surfaces such as paperboard cartons.